Bipolar selector with independently tunable threshold voltages

ABSTRACT

Various embodiments of the present application are directed towards a bipolar selector having independently tunable threshold voltages, as well as a memory cell comprising the bipolar selector and a memory array comprising the memory cell. In some embodiments, the bipolar selector comprises a first unipolar selector and a second unipolar selector. The first and second unipolar selectors are electrically coupled in parallel with opposite orientations and may, for example, be diodes or some other suitable unipolar selectors. By placing the first and second unipolar selectors in parallel with opposite orientations, the first unipolar selector independently defines a first threshold voltage of the bipolar selector and the second unipolar selector independently defines a second threshold voltage of the bipolar selector. As a result, the first and second threshold voltages can be independently tuned by adjusting parameters of the first and second unipolar selectors.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.17/230,222, filed on Apr. 14, 2021, which is a Continuation of U.S.application Ser. No. 16/411,706, filed on May 14, 2019 (now U.S. Pat.No. 10,991,756, issued on Apr. 27, 2021), which claims the benefit ofU.S. Provisional Application No. 62/749,210, filed on Oct. 23, 2018. Thecontents of the above-referenced applications are hereby incorporated byreference in their entirety.

BACKGROUND

Many modern-day electronic devices include electronic memory. Across-point memory architecture with one-selector one-resistor (1S1R)memory cells is increasingly receiving attention for use with nextgeneration electronic memory due to its high density. Examples of nextgeneration electronic memory include resistive random-access memory(RRAM), phase-change random-access memory (PCRAM), magnetoresistiverandom-access memory (MRAM), and conductive-bridging random-accessmemory (CBRAM).

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a schematic diagram of some embodiments of a memorycell comprising a bipolar selector with independently tunable thresholdvoltages.

FIGS. 2A and 2B illustrate schematic diagrams of various more detailedembodiments of the memory cell of FIG. 1 in which a data-storage elementof the memory cell is a magnetic tunnel junction (MTJ).

FIGS. 3A and 3B illustrate schematic diagrams of some alternativeembodiments of the memory cells respectively of FIGS. 2A and 2B in whichindividual selectors of the bipolar selector have different sizes.

FIGS. 4A and 4B illustrate schematic diagrams of some more detailedembodiments of the memory cells respectively of FIGS. 3A and 3B in whichthe individual selectors of the bipolar selector are multilayer stacks.

FIG. 5 illustrates a graph of some embodiments of current-voltage (I-V)curves for the bipolar selector of FIG. 1 .

FIGS. 6A and 6B illustrate cross-sectional views of various embodimentsof an integrated chip comprising the memory cell of FIG. 1 .

FIG. 7 illustrates a schematic view of some embodiments of a memoryarray comprising a plurality of memory cells, where the memory cellscomprise bipolar selectors having independently tunable thresholdvoltages.

FIGS. 8A-8C illustrate schematic views of some embodiments of the memoryarray of FIG. 7 at various operational states.

FIGS. 9A-9D illustrate schematic views of various alternativeembodiments of the memory array of FIG. 7 .

FIGS. 10A and 10B illustrate schematic views of various embodiments of athree-dimensional (3D) memory array comprising a plurality of memorycells, where the memory cells comprise bipolar selectors havingindependently tunable threshold voltages.

FIGS. 11A and 11B illustrate cross-sectional views of variousembodiments of an integrated chip comprising a pair of stacked memorycells respectively in FIGS. 10A and 10B.

FIGS. 12-17 illustrate a series of cross-sectional views of someembodiments of a method for forming an integrated chip comprising amemory array, where memory cells of the memory array comprise bipolarselectors with independently tunable threshold voltages.

FIG. 18 illustrates a block diagram of some embodiments of the method ofFIGS. 12-17 .

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples,for implementing different features of this disclosure. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

A cross-point memory array may, for example, comprise multipleone-selector one-resistor (1S1R) memory cells respectively at crosspoints of bit lines and word lines. By appropriately biasing a bit lineand a word line, a 1S1R memory cell at a cross point of the bit and wordlines is selected and current flows through the 1S1R memory cell. When a1S1R memory cell is selected, a selector of the 1S1R memory cell isbiased with a voltage greater than a threshold voltage of the selector.Further, selectors of unselected memory cells at a bit line of the 1S1Rmemory cell and selectors of unselected memory cells at a word line ofthe 1S1R memory cell are biased since the bit line and the word line areshared. However, voltages across the selectors of the unselected memorycells are less than threshold voltages of the selectors, whereby currentdoes not flow through the other 1S1R memory cells.

The 1S1R memory cells may be unipolar or bipolar. A unipolar 1S1R memorycell is read from and written to at a single polarity. A bipolar 1S1Rmemory cell is read from and/or written to at two polarities. Forexample, a bipolar 1S1R memory cell may be set to different statesrespectively at different polarities. Therefore, selectors for unipolar1S1R memory cells (i.e., unipolar selectors) switch at a single polarityand/or have a single threshold voltage, and selectors for bipolar 1S1Rmemory cells (i.e., bipolar selectors) switch at two polarities and/orhave multiple threshold voltages respectively at the two polarities.

Bipolar selectors typically have symmetrical threshold voltages. Asymmetrical threshold voltage has a similar value at a first polarity asat a second polarity and cannot be tuned for one polarity withoutsimilar tuning for the other polarity. However, bias voltages used at afirst polarity may be different than bias voltages used at a secondpolarity, whereby it may be difficult to properly match a symmetricalthreshold voltage to the bias voltages at both the first polarity andthe second polarity. Due to a poorly matching threshold voltage,unselected 1S1R memory cells sharing a bit or word line with a selected1S1R memory cell may have selectors that are not fully OFF. As a result,leakage current may flow through the unselected 1S1R memory cells andcause a read disturbance and/or a write disturbance. Further, due to apoorly matching threshold voltage, a selected 1S1R memory cell may havea selector that is not fully ON. As a result, the selector may cause ahigh amount of parasitic resistance in the selected 1S1R memory cellthat causes a read disturbance. A read disturbance may reduce a readwindow of a selected 1S1R memory cell and/or may cause read failure. Awrite disturbance may cause a state of an unselected 1S1R memory cell tochange.

Various embodiments of the present application are directed towards abipolar selector having independently tunable threshold voltages, aswell as a memory cell comprising the bipolar selector and a memory arraycomprising the memory cell. In some embodiments, the bipolar selectorcomprises a first unipolar selector and a second unipolar selector. Aunipolar selector may, for example, that switches at a single polarityand/or has a single threshold voltage, whereas a bipolar selector may,for example, be a selector that switches at multiple polarities and/orhas multiple threshold voltages respectively at the multiple polarities.The first and second unipolar selectors are electrically coupled inparallel with opposite orientations and may, for example, be diodes orsome other suitable unipolar selectors.

By placing the first and second unipolar selectors in parallel withopposite orientations, the first unipolar selector independently definesa first threshold voltage of the bipolar selector and the secondunipolar selector independently defines a second threshold voltage ofthe bipolar selector. As a result, the first and second thresholdvoltages can be independently tuned by adjusting parameters of the firstand second unipolar selectors. The independent tuning allows the firstand second threshold voltages to be better matched to bias conditionsfor reading from and/or writing to the memory cell when the polarity ofthe memory cell changes between read and/or write operations. By bettermatching the bias conditions, read disturbance and/or write disturbancemay be reduced.

With reference to FIG. 1 , a schematic diagram 100 of some embodimentsof a memory cell 102 comprising a bipolar selector 104 withindependently tunable threshold voltages is provided. The bipolarselector 104 is electrically coupled in series with a data-storageelement 106, from a bit line BL to a source line SL. In someembodiments, locations of the bit line BL and the source line SL arereversed. Further, the bipolar selector 104 has a first thresholdvoltage at a first polarity, and further has a second threshold voltageat a second polarity. In some embodiments, the first and secondthreshold voltages are different. For example, the first thresholdvoltage may be 5V, whereas the second threshold voltage may be 4V, orvice versa. Other values are, however, amenable for the first and secondthreshold voltages. In other embodiments, the first and second thresholdvoltages are the same.

The bipolar selector 104 is at the first polarity when the voltageacross the bipolar selector 104 is positive from the bit line BL to thedata-storage element 106, whereas the bipolar selector 104 is at thesecond polarity when the voltage across the bipolar selector 104 ispositive from the data-storage element 106 to the bit line BL. At thefirst polarity, the bipolar selector 104 conducts and/or is in a lowresistance state if the voltage across the bipolar selector 104, fromthe bit line BL to the data-storage element 106, exceeds the firstthreshold voltage. Otherwise, at the first polarity, the bipolarselector 104 is non-conducting and/or is in a high resistance state. Atthe second polarity, the bipolar selector 104 conducts and/or is in alow resistance state if the voltage across the bipolar selector 104,from the data-storage element 106 to the bit line BL, exceeds the secondthreshold voltage. Otherwise, at the second polarity, the bipolarselector 104 is non-conducting and/or is in a high resistance state.

The bipolar selector 104 comprises a first unipolar selector 108 and asecond unipolar selector 110. The first and second unipolar selectors108, 110 are electrically coupled in parallel with opposite directions.A unipolar selector is a device that switches at a single polarityand/or has a single threshold voltage. At a first polarity, the unipolarselector conducts and/or is in a low resistance state if the voltageacross the unipolar selector exceeds a threshold voltage. Otherwise, atthe first polarity, the unipolar selector is non-conducting and/or is ina high resistance state. At the second polarity, the unipolar selectoris non-conducting and/or in a high resistance state. The first andsecond unipolar selectors 108, 110 may, for example, have oppositedirections in that the first unipolar selector 108 is configured toselectively allow current to flow in a first direction while blockingthe flow of current in a second direction, whereas the second unipolarselector 110 is configured to selectively allow current to flow in thesecond direction while blocking the flow of current in the firstdirection. The first and second unipolar selectors 108, 110 may, forexample, be PIN diodes, polysilicon diodes, punch-through diodes,varistor-type selectors, ovonic threshold switches (OTS s),doped-chalcogenide-based selectors, Mott effect based selectors,mixed-ionic-electronic-conductive (MIEC)-based selectors,field-assisted-superliner-threshold (FAST) selectors, filament-basedselectors, doped-hafnium-oxide-based selectors, or some other suitablediodes and/or selectors.

By placing the first and second unipolar selectors 108, 110 in parallelwith opposite directions, the first unipolar selector 108 independentlydefines the first threshold voltage and the second unipolar selector 110independently defines the second threshold voltage. As a result, thefirst and second threshold voltages can be independently tuned byadjusting parameters of the first and second unipolar selectors 108,110. The independent tuning allows the first and second thresholdvoltages to be better matched to bias conditions for reading from and/orwriting to the memory cell 102 when the polarity of the memory cell 102changes between read and/or write operations. By better matching thebias conditions, read disturbance may be reduced while reading thememory cell 102. Further, write disturbance to neighboring memory cells(not shown) may be reduced while reading from and/or writing to thememory cell 102.

In some embodiments, the first unipolar selector 108 has only twoterminals and/or the second unipolar selector 110 has only twoterminals. In some embodiments, the first unipolar selector 108 has morethan two terminals and/or the second unipolar selector 110 has more thantwo terminals. In some embodiments (e.g., where the first and secondunipolar selectors 108, 110 are diodes), a cathode of the first unipolarselector 108 is electrically coupled to an anode of the second unipolarselector 110 and an anode of the first unipolar selector 108 iselectrically coupled to a cathode of the second unipolar selector 110.In alternative embodiments, bipolar selectors and/or some other suitabletype of selectors are used in place of unipolar selectors for the firstand second unipolar selectors 108, 110.

The data-storage element 106 stores a bit of data. In some embodiments,a resistance of the data-storage element varies depending upon a datastate of the data-storage element 106. For example, the data-storageelement 106 may have a low resistance at a first data state and may havea high resistance at a second data state. In other embodiments,capacitance or some other suitable parameter of the data-storage element106 varies depending upon a data state of the data-storage element 106.In some embodiments, the data-storage element 106 is a magnetic tunneljunction (MTJ) or some other suitable magnetic junction and/or thememory cell 102 is a spin-transfer torque magnetic random-access memory(STT-MRAM) cell or some other suitable MRAM cell. In some embodiments,the data-storage element 106 is a metal-insulator-metal (MIM) stackand/or the memory cell 102 is a resistance random-access memory (RRAM)cell. Other structures for the data-storage element 106 and/or othermemory-cell types for the memory cell 102 are, however, amenable.

In some embodiments, the data-storage element 106 is set to a first datastate at the first polarity and is set to a second data state at thesecond polarity, such that writing to the data-storage element 106 isbipolar. For example, where the data-storage element 106 is an MTJ, thedata-storage element 106 may be set to a first data state at the firstpolarity and may be set to a second data state at the second polarity.Accordingly, the first threshold voltage is used while setting thedata-storage element 106 to the first data state, whereas the secondthreshold voltage is used while setting the data-storage element 106 tothe second data state. In some embodiments, the data-storage element 106is read from at the first polarity, such that reading from thedata-storage element 106 is unipolar. Accordingly, only the firstthreshold voltage is used while reading from the data-storage element106.

With reference to FIG. 2A, a schematic diagram 200A of some moredetailed embodiments of the memory cell 102 of FIG. 1 is provided inwhich the data-storage element 106 is an MTJ. The data-storage element106 comprises a reference element 202, a free element 204, and a barrierelement 206. The barrier element 206 is non-magnetic and is sandwichedbetween the reference and free elements 202, 204. The reference and freeelements 202, 204 are ferromagnetic, and the free element 204 overliesthe reference element 202 and the barrier element 206. Further, thereference element 202 has a fixed magnetization, whereas the freeelement 204 has variable a magnetization.

Depending upon whether magnetizations of the reference and free elements202, 204 are parallel or anti-parallel, the data-storage element 106 hasa low resistance or a high resistance. For example, the data-storageelement 106 may have the low resistance when the magnetizations of thereference and free elements 202, 204 are parallel and may have the highresistance when the magnetizations are antiparallel. The low and highresistances may, in turn, be used to represent different data states ofthe data-storage element 106.

A first write voltage is applied across the data-storage element 106 ata first polarity to set the data-storage element 106 to an antiparallelstate, and a second write voltage is applied across the data-storageelement 106 at a second polarity to set the data-storage element 106 toa parallel state. In some embodiments, the second write voltage isgreater than the first write voltage since the data-storage element 106is typically, but not always, in a high resistance state (i.e., theantiparallel state) when setting the data-storage element 106 to theparallel state. The second write voltage may, for example, be about1.5-3.0 times greater than the first write voltage. However, othermultiples greater than one (e.g., 5.0 or some other value) are amenable.In some embodiments in which the second write voltage is greater thanthe first write voltage, the second threshold voltage is greater thanthe first threshold voltage since the first threshold voltage is used atthe same polarity as the first write voltage and the second thresholdvoltage is used at the same polarity as the second write voltage. Thismay, for example, be schematically illustrated by an increased length L₂of the second unipolar selector 110 compared to a length L₁ of the firstunipolar selector 108.

In some embodiments, the barrier element 206 is a tunnel barrierselectively allowing quantum mechanical tunneling of electrons throughthe barrier element 206. For example, quantum mechanical tunneling maybe allowed when the reference and free elements 202, 204 have parallelmagnetizations, and may be blocked when the reference and free elements202, 204 have antiparallel magnetizations. The barrier element 206 may,for example, be or comprise an amorphous barrier, a crystalline barrier,or some other suitable barrier. The amorphous barrier may be orcomprise, for example, aluminum oxide (e.g., AlO_(x)), titanium oxide(e.g., TiO_(x)), or some other suitable amorphous barrier. Thecrystalline barrier may be or comprise manganese oxide (e.g., MgO),spinel (e.g., MgAl₂O₄), or some other suitable crystalline barrier.

In some embodiments, the reference element 202 is or comprises cobaltiron (e.g., CoFe), cobalt iron boron (e.g., CoFeB), or some othersuitable ferromagnetic material(s), or any combination of the foregoing.In some embodiments, the reference element 202 adjoins anantiferromagnetic element (not shown) and/or is part of or otherwiseadjoins a synthetic antiferromagnetic (SAF) element (not shown). In someembodiments, the free element 204 is or comprises cobalt iron (e.g.,CoFe), cobalt iron boron (e.g., CoFeB), or some other suitableferromagnetic material(s), or any combination of the foregoing.

With reference to FIG. 2B, a schematic diagram 200B of some alternativeembodiments the memory cell 102 of FIG. 2A is provided in which thereference element 202 overlies the free element 204. Since the referenceelement 202 overlies the free element 204, the polarities at which thefirst and second write voltages are applied across the data-storageelement 106 are reversed compared to FIG. 2A. The first write voltage isapplied across the data-storage element 106 at the second polarity toset the data-storage element 106 to the antiparallel state, and thesecond write voltage is applied across the data-storage element 106 atthe first polarity to set the data-storage element 106 to the parallelstate. In some embodiments in which the second write voltage is greaterthan the first write voltage, the first threshold voltage is greaterthan the second threshold voltage since the first threshold voltage isused at the same polarity as the second write voltage and the secondthreshold voltage is used at the same polarity as the first writevoltage. This may, for example, be schematically illustrated by anincreased length L₁ of the first unipolar selector 108 compared to thelength L₂ of the second unipolar selector 110.

With reference to FIG. 3A, a schematic diagram 300A of some alternativeembodiments of the memory cell 102 of FIG. 2A is provided in which asize of the first unipolar selector 108 is greater than a size of thesecond unipolar selector 110. This may, for example, be schematicallyillustrated by an increased width W₁ of the first unipolar selector 108compared to a width W₂ of the second unipolar selector 110. In someembodiments, the first and second unipolar selectors 108, 110 are eachformed by a multilayer stack and the size of the first unipolar selector108 is greater than the size of the second unipolar selector 110 interms of cross-sectional width of the multilayer stack.

In some embodiments, the first unipolar selector 108 has a lesser ONresistance than an ON resistance of the second unipolar selector 110 dueto the greater size. Further, in some embodiments, the first unipolarselector 108 is ON while reading the memory cell 102, whereas the secondunipolar selector 110 is OFF while reading the memory cell 102. Hence,the greater size of the first unipolar selector 108 may reduce parasiticresistance while reading the memory cell 102, which enlarges the readwindow.

With reference to FIG. 3B, a schematic diagram 300B of some alternativeembodiments of the memory cell 102 of FIG. 2B is provided in which asize of the second unipolar selector 110 is greater than a size of thefirst unipolar selector 108. This may, for example, be schematicallyillustrated by an increased width W₂ of the second unipolar selector 110compared to the width W₁ of the first unipolar selector 108. The greatersize of the second unipolar selector 110 may, for example, reduceparasitic resistance while reading the memory cell 102, which enlargesthe read window and reduces read disturbance.

With reference to FIG. 4A, a schematic diagram 400A of some moredetailed embodiments of the memory cell 102 of FIG. 3A is provided inwhich the first and second unipolar selectors 108, 110 are multilayerstacks. The first and second unipolar selectors 108, 110 compriseindividual cathodes 402, individual insulators 404, and individualanodes 406. The insulators 404 are each sandwiched between a respectiveone of the cathodes 402 and a respective one of the anodes 406. Themultilayer stacks may, for example, be or comprise PIN diodes, MIMdevices, or some other multilayer devices.

In some embodiments in which the multilayer stacks are PIN diodes, thecathodes 402 are or comprise N-type semiconductor material, the anodes406 are or comprise P-type semiconductor material, and the insulators404 are or comprise intrinsic or lightly doped semiconductor material.The insulators 404 may, for example, be lightly doped relative to thecathodes 402 and/or the anodes 406. The semiconductor material of themultilayer stacks may, for example, be or comprises polysilicon,monocrystalline silicon, germanium, indium gallium arsenide, or someother suitable semiconductor material. In some embodiments in which themultilayer stacks are MIM devices, the cathodes 402 and the anodes 406are or comprise metal or some other suitable conductive material and/orthe insulators 404 are or comprise doped hafnium oxide, some othersuitable metal oxide, or some other suitable insulator material.

In some embodiments, thicknesses of the insulators 404 are varied toadjust threshold voltages of the first and second unipolar selectors108, 110. For example, increasing a thickness of an insulator mayincrease a threshold voltage of the corresponding unipolar selectorwhereas decreasing the thickness may decrease the threshold voltage. Insome embodiments, a second insulator thickness T₂ of the second unipolarselector 110 is greater than a first insulator thickness T₁ of the firstunipolar selector 108 so the second unipolar selector 110 has a greaterthreshold voltage than the first unipolar selector 108. In someembodiments, doping concentrations of the insulators 404 are varied toadjust threshold voltages of the first and second unipolar selectors108, 110. For example, increasing a doping concentration of an insulatormay decrease a threshold voltage of the corresponding selector whereasdecreasing the doping concentration may increase the threshold voltage.

In some embodiments, widths of the of the first and second unipolarselectors 108, 110 are varied to adjust ON resistances of the first andsecond unipolar selectors 108, 110. For example, increasing a width of aselector may decrease an ON resistance of the selector whereasdecreasing the width may increase the ON resistance. In someembodiments, a second width W₂ of the second unipolar selector 110 isless than a first width W₁ of the first unipolar selector 108 so thefirst unipolar selector 108 has a lesser ON resistance than the secondunipolar selector 110. As noted above, the lesser ON resistance mayenlarge the read window for the memory cell 102 when the first unipolarselector 108 is ON during reads.

With reference to FIG. 4B, a schematic diagram 400B of some moredetailed embodiments of the memory cell 102 of FIG. 3B is provided inwhich the first and second unipolar selectors 108, 110 are multilayerstacks. The first and second unipolar selectors 108, 110 compriseindividual cathodes 402, individual insulators 404, and individualanodes 406. The cathodes 402, the insulators 404, and the anodes 406may, for example, be as described above with regard to FIG. 4A. In someembodiments, the first insulator thickness T₁ of the first unipolarselector 108 is greater than the second insulator thickness T₂ of thesecond unipolar selector 110 so the first unipolar selector 108 has agreater threshold voltage than the second unipolar selector 110. In someembodiments, the second width W₂ of the second unipolar selector 110 isgreater than the first width W₁ of the first unipolar selector 108 sothe second unipolar selector 110 has a lesser ON resistance than thefirst unipolar selector 108.

With reference to FIG. 5 , a graph 500 of some embodiments ofcurrent-voltage (I-V) curves for the bipolar selector 104 of FIG. 1 isprovided. A horizontal axis of the graph 500 corresponds to voltage, anda vertical axis of the graph 500 corresponds to current. Further, atop-right quadrant of the graph 500 corresponds to a first polarity ofthe bipolar selector 104, and a bottom-left quadrant of the graph 500corresponds to a second polarity of the bipolar selector 104. The graph500 includes a first I-V curve 502 and a second I-V curve 504.

Focusing on the first I-V curve 502, current is about zero until thevoltage exceeds a first threshold voltage V_(T1) of the bipolar selector104 and then increases in magnitude with voltage. Further, current isabout zero until the voltage exceeds a second threshold voltage V_(T2)of the bipolar selector 104 and then increases in magnitude withvoltage. In some embodiments, the bipolar selector 104 of FIG. 1 has thefirst I-V curve 502 when the first and second unipolar selectors 108,110 of FIG. 1 are polysilicon diodes, PIN diodes, or some other suitabletype of diode. For example, embodiments of the bipolar selectors 104 inFIGS. 4A and 4B may have the first I-V curve 502 since the first andsecond unipolar selectors 108, 110 may be PIN diodes.

Focusing on the second I-V curve 504, the second I-V curve 504 has asnapback shape. Current is about zero until the voltage exceeds thefirst threshold voltage V_(T1) of the bipolar selector 104 and thenincreases in magnitude. As the current increases in magnitude, thevoltage snaps back towards zero volts before increasing in magnitude.Further, current is about zero until the voltage exceeds the secondthreshold voltage V_(T2) of the bipolar selector 104 and then increasesin magnitude. As the current increases in magnitude, the voltage snapsback towards zero volts before increasing in magnitude. In someembodiments, the bipolar selector 104 has the second I-V curve 504 whenthe first and second unipolar selectors 108, 110 are MIM devicescomprising doped-hafnium-oxide insulators. For example, embodiments ofthe bipolar selector 104 in FIGS. 4A and 4B may have the second I-Vcurve 504 since the first and second unipolar selectors 108, 110 may beMIM devices comprising doped-hafnium-oxide insulators.

Embodiments of the bipolar selector 104 having the second I-V curve 504may, for example, have a lesser ON resistance compared to embodiments ofthe bipolar selector 104 having the first I-V curve 502. For a givencurrent (demarcated by dashed line 506), the second I-V curve 504 has alesser voltage than the first I-V curve 502 due to the snapback. Hence,by Ohm's law, the resistance across the bipolar selector 104 is less forembodiments of the bipolar selector 104 having the second I-V curve 504than embodiments of the bipolar selector 104 having the first I-V curve502. The lesser resistance, in turn, enlarges the read window for thememory cell 102 of FIG. 1 since there is less parasitic resistance.

Regardless of whether the bipolar selector 104 has the first or secondI-V curve 502, 504, the first threshold voltage V_(T1) is defined by thefirst unipolar selector 108 of FIG. 1 and the second threshold voltageV_(T2) is defined by the second unipolar selector 110 of FIG. 1 . Insome embodiments, the first and second threshold voltages V_(T1), V_(T2)have different magnitudes, such that the bipolar selector 104 has anasymmetric threshold voltage. In some embodiments, the first and secondthreshold voltages V_(T1), V_(T2) have the same magnitude, such that thebipolar selector 104 has a symmetric threshold voltage.

With reference to FIG. 6A, a cross-sectional view 600A of someembodiments of an integrated chip comprising the memory cell 102 of FIG.1 is provided. The memory cell 102 overlies a substrate 602 and islocated within an interconnect structure 604 that covers the substrate602. The interconnect structure 604 comprises an interconnect dielectriclayer 606, a plurality of wires 608, and a plurality of vias 610. Forease of illustration, only some of the vias 610 are labeled 610. Theinterconnect dielectric layer 606 accommodates the wires 608, the vias610, and the memory cell 102 and may, for example, be or comprisesilicon oxide, a low κ dielectric, some other suitable dielectric(s), orany combination of the foregoing. As used herein, a low κ dielectric maybe, for example, a dielectric with a dielectric constant κ less thanabout 3.9, 3, 2, or 1.

The wires 608 and the vias 610 are alternatingly stacked in theinterconnect dielectric layer 606 to define conductive pathsinterconnecting components of the memory cell 102 and/or connecting thememory cell 102 to other devices (not shown) in the integrated chip. Forexample, the wires 608 and the vias 610 may define conductive pathselectrically coupling the first and second unipolar selectors 108, 110in parallel. As another example, the wires 608 and the vias 610 maydefine conductive paths electrically coupling the bipolar selector 104in series with the data-storage element 106 from a wire defining the bitline BL to a wire defining the source line SL. The wires 608 and thevias 610 may, for example, be or comprise metal, some other suitableconductive material(s), or any combination of the foregoing.

With reference to FIG. 6B, a cross-sectional view 600B of somealternative embodiments of the integrated chip of FIG. 6A is provided inwhich a semiconductor device 612 underlies the memory cell 102.Arranging the semiconductor device 612 under the memory cell 102 may,for example, enhance a functional density of the integrated chip. Insome embodiments, the semiconductor device 612 is electrically separatefrom the memory cell 102 and/or the wires 608 and the vias 610 do notdefine a conductive path directly from the semiconductor device 612 tothe memory cell 102. In other embodiments, the semiconductor device 612is electrically coupled to the memory cell 102 by the wires 608 and thevias 610. The semiconductor device 612 may, for example, be ametal-oxide-semiconductor (MOS) device, an insulated-gate field-effecttransistor (IGFET), or some other suitable semiconductor device.

In some embodiments, the semiconductor device 612 comprises a pair ofsource/drain regions 614, a gate dielectric layer 616, and a gateelectrode 618. The source/drain regions 614 are in the substrate 602,along a top surface of the substrate 602. The gate dielectric layer 616and the gate electrode 618 are stacked over the substrate 602,vertically between the substrate 602 and the interconnect structure 604and laterally between the source/drain regions 614.

With reference to FIG. 7 , a schematic view 700 of some embodiments of amemory array 702 comprising a plurality of memory cells 102 in aplurality of rows and a plurality of columns is provided. For ease ofillustration, only some of the memory cells 102 are labeled 102. In someembodiments, only a portion of the memory array 702 is illustrated. Forexample, despite the three illustrated rows and the three illustratedcolumns, more rows and more columns may be present outside the schematicview 700. In other embodiments, the memory array 702 is fullyillustrated and hence has three rows and three columns.

The memory cells 102 comprise individual bipolar selectors 104 havingindependently tunable threshold voltages, and further comprisingindividual data-storage elements 106. For ease of illustration, onlysome of the bipolar selectors 104 are labeled 104, and only some of thedata-storage elements 106 are labeled 106. The bipolar selectors 104 areelectrically coupled in series with the data-storage elements 106,respectively, and comprise individual first unipolar selectors 108 andindividual second unipolar selectors 110. For clarity, only some of thefirst unipolar selectors 108 are labeled 108, and only some of thesecond unipolar selectors 110 are labeled 110. The first unipolarselectors 108 are electrically coupled in parallel with the secondunipolar selectors 110, respectively, and define threshold voltages ofthe bipolar selectors 104 at a first polarity. The second unipolarselectors 110 define threshold voltages of the bipolar selectors 104 ata second polarity. The memory cells 102 may, for example, each be asillustrated and described with regard to FIG. 1 .

Bit lines extend laterally along corresponding rows of the memory arrayand electrically couple with memory cells in the corresponding rows,whereas source lines extend laterally along corresponding columns of thememory array and electrically couple with memory cells in thecorresponding columns. For clarity, the bit lines are respectivelylabeled BL_(m), BL_(m+1), and BL_(m+2), where the subscripts identifycorresponding rows and m is an integer variable representing a row inthe memory array 702. Similarly, for clarity, the source lines arerespectively labeled SL_(n), SL_(n+1), and SL_(n+2), where thesubscripts identify corresponding columns and n is an integer variablerepresenting a column in the memory array.

By appropriately biasing a bit line and a source line, the memory cellat the cross point of the bit line and the source line may be selectedand read from or written to. In some embodiments, the bias conditionshave different polarities depending upon whether writing a first datastate to a memory cell or a second data state to a memory cell. Further,the bipolar selectors 104 prevent read and/or write disturbance tounselected memory cells sharing a bit line or a source line with theselected memory cell.

With reference to FIG. 8A-8C, schematic diagram 800A-800C of someembodiments of the memory array 702 of FIG. 7 are provided at variousoperational states to illustrate operation of the bipolar selectors 104.FIG. 8A illustrates the memory array 702 while writing a selected memorycell 102 s to a first data state (e.g., a logic “1”), and FIG. 8Billustrates the memory array 702 while writing the selected memory cell102 s to a second data state (e.g., a logic “0”). FIG. 8C illustratesthe memory array 702 while reading a state of the selected memory cell102 s.

As illustrated by FIG. 8A, the selected memory cell 102 s is at thecross point of source line SL_(n) and bit line BL_(m+2). Bit lineBL_(m+2) is biased with a first write voltage V_(w1) while source lineSL_(n) is grounded. In some embodiments, the other source linesSL_(n+1), SL_(n+2) and the other bit lines BL_(m), BL_(m+1) are biasedwith half the first write voltage V_(w1) or some other fraction of thefirst write voltage V_(w1) to reduce write disturbance to unselectedmemory cells. The first write voltage V_(w1) is positive from bit lineBL_(m+2) to source SL_(n), such that the selected memory cell 102 s isat a first polarity and the second unipolar selector 110 of the selectedmemory cell 102 s is OFF. Further, the first write voltage V_(w1)exceeds a first threshold voltage of the first unipolar selectors 108,such that the first unipolar selector 108 of the selected memory cell102 s is ON and current I_(w1) flows through the selected memory cell102 s. The current I_(w1), in turn, sets the data-storage element 106 ofthe selected memory cell 102 s to the first data state.

Some unselected memory cells 102 u (only some of which are labeled 102u) share source line SL_(n) and bit line BL_(m+2) with the selectedmemory cell 102 s, whereby these unselected memory cells 102 u are alsobiased at the first polarity. For example, the unselected memory cells102 u may be biased with a voltage that is about half the first writevoltage V_(w1). However, the bias voltages of the unselected memorycells 102 u are less than the first threshold voltage of the firstunipolar selectors 108, whereby the first unipolar selectors 108 of theunselected memory cells 102 u are OFF. Further, since the unselectedmemory cells 102 u are biased at the first polarity, the second unipolarselectors 110 of the unselected memory cells 102 u are OFF. Accordingly,current does not flow through the unselected memory cells 102 u andthere is no write disturbance to the unselected memory cells 102 u.

As illustrated by FIG. 8B, source line SL_(n) is biased with a secondwrite voltage V_(w0) while bit line is BL_(m+2) is grounded. In someembodiments, the other source lines SL_(n+1), SL_(n+2) and the other bitlines BL_(m), BL_(m+1) are biased with half the second write voltageV_(w0) or some other fraction of the second write voltage V_(w0). Thesecond write voltage V_(w0) is positive from source line SL_(n) to bitline BL_(m+2), such that the selected memory cell 102 s is at a secondpolarity and the first unipolar selector 108 of the selected memory cell102 s is OFF. Further, the second write voltage V_(w0) exceeds a secondthreshold voltage of the second unipolar selectors 110, such that thesecond unipolar selector 110 of the selected memory cell 102 s is ON andcurrent V_(w0) flows through the selected memory cell 102 s. The currentV_(w0), in turn, sets the data-storage element 106 of the selectedmemory cell 102 s to the second data state.

The unselected memory cells 102 u sharing source line SL_(n) and bitline BL_(m+2) with the selected memory cell 102 s are also biased at thesecond polarity. For example, the unselected memory cells 102 u may bebiased with a voltage that is about half the second write voltageV_(w0). However, the bias voltages of the unselected memory cells 102 uare less than the second threshold voltage of the second unipolarselectors 110, whereby the second unipolar selectors 110 of theunselected memory cells 102 u are OFF. Further, since the unselectedmemory cells 102 u are biased at the second polarity, the first unipolarselectors 108 of the unselected memory cells 102 u are OFF. Accordingly,current does not flow through the unselected memory cells 102 u andthere is no write disturbance to the unselected memory cells 102 u.

As illustrated by FIG. 8C, bit line BL_(m+2) is biased with a readvoltage V_(r) while source line SL_(n) is grounded. FIG. 8C is as FIG.8A is described except that the read voltage V_(r) is used in place ofthe first write voltage V_(w1) and is small enough that the resultingread current I_(r) does not change a state of the selected memory cell102 s. When resistances of the data-storage elements 106 vary withcorresponding data states, the selected memory cell 102 s is in thefirst data state or the second data state depending upon the extent ofthe read current I_(r).

In some embodiments, the first and second write voltages V_(w1), V_(w0)and the read voltage V_(r) are different, whereby the first and secondthreshold voltages of the bipolar selectors 104 are different toproperly match the bias conditions during the different operations. Aproperly matching threshold voltage for the second polarity may, forexample, be a voltage halfway between: 1) a voltage across the bipolarselector 104 of the selected memory cell 102 s during the second writeoperation (see FIG. 8B); and 2) a voltage across the bipolar selectors104 of the unselected memory cells 102 u during the second writeoperation (see FIG. 8B). Since the first write operation and the readoperation are both performed at the first polarity, the bias conditionsduring both operations may be considered when properly selecting thefirst threshold voltage. A properly matching threshold voltage for thefirst polarity may, for example, be a voltage halfway between: 1) avoltage across the bipolar selector 104 of the selected memory cell 102s during the read operation (see FIG. 8C); and 2) a voltage across thebipolar selectors 104 of the unselected memory cells 102 u during thefirst write operation (see FIG. 8A). The first threshold voltage may,for example, be independently tuned relative to the second thresholdvoltage and vice versa because the first threshold voltage is set by thefirst unipolar selectors 108 and the second threshold voltage is areseparately defined by the second unipolar selectors 110.

With reference to FIGS. 9A, a schematic view 900A of some alternativeembodiments of the memory array 702 of FIG. 7 is provided in which thememory cells 102 are each as illustrated and described with regard toFIG. 2A. The data-storage elements 106 comprise individual referenceelements 202, individual free elements 204, and individual barrierelements 206. For ease of illustration, only some of the referenceelements 202 are labeled 202, only some of the free elements 204 arelabeled 204, and only some of the barrier elements 206 are labeled 206.The free elements 204 overlie the reference elements 202, and thebarrier elements are sandwiched between the reference elements 202 andthe free elements 204.

With reference to FIGS. 9B, a schematic view 900B of some alternativeembodiments of the memory array 702 of FIG. 9A is provided in which thememory cells 102 are each as illustrated and described with regard toFIG. 2B instead of FIG. 2A. With reference to FIG. 9C, a schematic view900C of some alternative embodiments of the memory array 702 of FIG. 9Ais provided in which the memory cells 102 are each as illustrated anddescribed with regard to FIG. 3A instead of FIG. 2A. With reference toFIG. 9D, a schematic view 900D of some alternative embodiments of thememory array 702 of FIG. 9A is provided in which the memory cells 102are each as illustrated and described with regard to FIG. 3B instead ofFIG. 2A.

With reference to FIG. 10A, a schematic view 1000A of some embodimentsof a three-dimensional (3D) memory array comprising a first memory array702 a and a second memory array 702 b is provided. The first and secondmemory arrays 702 a, 702 b are stacked, such that the second memoryarray 702 b overlies and is spaced from the first memory array 702 a.Stacking the first and second memory arrays 702 a, 702 b may, forexample, enhance memory density. In some embodiments, as illustrated,the first and second memory arrays 702 a, 702 b are each as the memoryarray 702 in FIG. 7 is illustrated and described. In other embodiments,the first and second memory arrays 702 a, 702 b are each as the memoryarray 702 in any one of FIGS. 9A-9D is illustrated and described. In yetother embodiments, the first memory array 702 a is as the memory array702 in one of FIGS. 7 and 9A-9D is illustrated and described, and thesecond memory array 702 b is as the memory array 702 in another one ofFIGS. 7 and 9A-9D is illustrated and described.

With reference to FIG. 10B, a schematic view 1000B of some alternativeembodiments the 3D memory array of FIG. 10A is provided in which thefirst and second memory arrays 702 a, 702 b share source lines. Asabove, the source lines are respectively labeled SL_(n), SL_(n+1), andSL_(n+2), where the subscripts identify corresponding columns and n isan integer variable representing a column in the 3D memory array.

With reference to FIG. 11A, a cross-sectional view 1100A of someembodiments of an integrated chip comprising a pair of stacked memorycells 102 from the 3D memory array of FIG. 10A is provided. The stackedmemory cells 102 are at the same row and the same column in the 3Dmemory array. Further, a lower one of the stacked memory cells 102 is inthe first memory array 702 a of FIG. 10A, whereas an upper one of thestacked memory cells 102 is in the second memory array 702 b of FIG.10A. The stacked memory cells 102 overlie a substrate 602 and aresurrounded by an interconnect dielectric layer 606 of an interconnectstructure 604. Further, wires 608 and vias 610 in the interconnectdielectric layer 606 electrically interconnect components of the stackedmemory cells 102.

With reference to FIG. 11B, a cross-sectional view 1100B of somealternative embodiments of the integrated chip of FIG. 11A is providedin which the stacked memory cells 102 are instead from the 3D memoryarray of FIG. 10B. Accordingly, the stacked memory cells 102 share asource line SL defined by one of the wires 608.

With reference to FIGS. 12-17 , a series of cross-sectional views1200-1700 of some embodiments of a method for forming an integrated chipcomprising a memory array is provided in which memory cells of thememory array comprise bipolar selectors with independently tunablethreshold voltages. For ease of illustration, the cross-sectional views1200-1700 only illustrate a first memory cell of the memory array.However, other memory cells of the memory array may, for example, beformed simultaneously with the first memory cell and/or each of theother memory cells may, for example, be formed as illustrated for thefirst memory cell.

As illustrated by the cross-sectional view 1200 of FIG. 12 , aninterconnect structure 604 is partially formed over a substrate 602. Thesubstrate 602 may, for example, be a bulk silicon substrate, asilicon-on-insulator (SOI) substrate, or some other suitable substrate.The interconnect structure 604 comprises a first interconnect dielectriclayer 606 a, a first wire 608 a defining a bit line BL, and a first setof vias 610 a. The first interconnect dielectric layer 606 aaccommodates the first wire 608 a and the first vias 610 a and may, forexample, be or comprise silicon oxide, a low κ dielectric, some othersuitable dielectric(s), or any combination of the foregoing. A low κdielectric may be, for example, a dielectric with a dielectric constantκ less than about 3.9, 3, 2, or 1. The first wire 608 a and the firstvias 610 a are stacked in the first interconnect dielectric layer 606 a,such that the first vias 610 a overlie the first wire 608 a.

In some embodiments, semiconductor devices (not shown) are on thesubstrate 602, between the substrate 602 and the interconnect structure604. In some embodiments, additional wires (not shown) and/or additionalvias (not shown) are alternatingly stacked in the first interconnectdielectric layer 606 a, between the substrate 602 and/or the first wire608 a. The additional wires and/or the additional vias may, for example,define conductive paths leading from semiconductor devices (not shown)on the substrate 602. In some embodiments, a process for partiallyforming the interconnect structure 604 comprises: 1) depositing a lowerinterconnect portion of the first interconnect dielectric layer 606 a onthe substrate 602; 2) forming the first wire 608 a inset into the lowerinterconnect portion; 3) forming an upper interconnect portion of thefirst interconnect dielectric layer 606 a on the first wire 608 a andthe lower interconnect portion; and 4) forming the first vias 610 ainset into the upper interconnect portion. Other processes for partiallyforming the interconnect structure 604 are, however, amenable.

As illustrated by the cross-sectional view 1300 of FIG. 13 , a firstunipolar selector 108 is formed overlying the bit line BL andelectrically coupled to the bit line BL by one of the first vias 610 a.The first unipolar selector 108 comprises a cathode 402 a, an insulator404 a, and an anode 406 a. The insulator 404 a is between the cathode402 a and the anode 406 a, and the cathode 402 a overlies the anode 406a. The cathode 402 a, the insulator 404 a, and the anode 406 a may, forexample, define a PIN diode, a MIM device, or some other multilayerdevice.

In some embodiments in which the cathode 402 a, the insulator 404 a, andthe anode 406 a define a PIN diode, the cathode 402 a is or compriseN-type semiconductor material, the anode 406 a is or comprises P-typesemiconductor material, and the insulator 404 a is or comprise intrinsicor lightly doped semiconductor material. The insulator 404 a may, forexample, be lightly doped relative to the cathode 402 a and/or the anode406 a. The semiconductor material for the cathode 402 a, the insulator404 a, and the anode 406 a may, for example, be or comprisespolysilicon, monocrystalline silicon, or some other suitablesemiconductor material. In some embodiments in which the cathode 402 a,the insulator 404 a, and the anode 406 a define a MIM device, thecathode 402 a and the anode 406 a are or comprise metal or some othersuitable conductive material and/or the insulator 404 a is or comprisesdoped hafnium oxide, some other suitable metal oxide, or some othersuitable insulator material.

In some embodiments, a process for forming the first unipolar selector108 comprises: 1) depositing an anode layer on the interconnectstructure 604; 2) depositing an insulator layer on the anode layer; 3)depositing a cathode layer on the insulator layer; and 4) patterning themultilayer film into the first unipolar selector 108. Other processesare, however, amenable. The depositing may, for example, be performed bychemical vapor deposition (CVD), physical vapor deposition (PVD),electroless plating, electroplating, some other suitable depositionprocess(es), or any combination of the foregoing. The patterning may,for example, be performed by a photolithography/etching process and/orsome other suitable patterning process(es).

As illustrated by the cross-sectional view 1400 of FIG. 14 , a secondunipolar selector 110 is formed overlying the bit line BL, adjacent tothe first unipolar selector 108 and electrically coupled to the bit lineBL by another one of the first vias 610 a. Similar to the first unipolarselector 110, the second unipolar selector 110 comprises a cathode 402b, an insulator 404 b, and an anode 406 b, and the insulator 404 b isbetween the cathode 402 b and the anode 406 b. However, in contrast withthe first unipolar selector 108, the anode 406 b overlies the cathode402 b and the second unipolar selector 110 has a different orientationthan the first unipolar selector 108. The cathode 402 b, the insulator404 b, and the anode 406 b may, for example, define a PIN diode, a MIMdevice, or some other multilayer device.

In some embodiments in which the cathode 402 b, the insulator 404 b, andthe anode 406 b define a PIN diode, the cathode 402 b, the insulator 404b, and the anode 406 b are as corresponding counterparts are describedfor the first unipolar selector 108. In some embodiments in which thecathode 402 b, the insulator 404 b, and the anode 406 b define a MIMdevice, the cathode 402 b, the insulator 404 b, and the anode 406 b areas corresponding counterparts are described for the first unipolarselector 108.

In some embodiments, the insulator 404 b of the second unipolar selector110 has a greater thickness than the insulator 404 a of the firstunipolar selector 108 so the second unipolar selector 110 has a greaterthreshold voltage than the first unipolar selector 108. The differencesbetween threshold voltages of the first and second unipolar selectors108, 110 may, for example, enable the first and second unipolarselectors 108, 110 to better match corresponding bias conditions toreduce read and/or write disturbance. In some embodiments, the first andsecond unipolar selectors 110 have different widths.

In some embodiments, a process for forming the second unipolar selector110 comprises: 1) depositing a cathode layer on the interconnectstructure 604; 2) depositing an insulator layer on the anode layer; 3)depositing an anode layer on the insulator layer; and 4) patterning themultilayer film into the first unipolar selector 108. Other processesare, however, amenable. The depositing and the patterning may, forexample, be as described for the first unipolar selector 108.

As illustrated by the cross-sectional view 1500 of FIG. 15 , theinterconnect structure 604 is extended around the first and secondunipolar selectors 108, 110, such that the interconnect structure 604electrically couples the first and second unipolar selectors 108, 110 inparallel to define a bipolar selector 104. The extended interconnectstructure 604 further comprises a second interconnect dielectric layer606 b, a second wire 608 b, and a set of second vias 610 b. The secondinterconnect dielectric layer 606 b accommodates the second wire 608 band the second vias 610 b, and may, for example, be as the firstinterconnect dielectric layer 606 a is described. The second wire 608 band the second vias 610 b are stacked in the second interconnectdielectric layer 606 b, such that the second wire 608 b is electricallycoupled to the first and second unipolar selectors 108, 110 by some ofthe second vias 610 b and one of the second vias 610 b overlies thesecond wire 608 b.

In some embodiments, a process for extending the interconnect structure604 comprises: 1) depositing a lower interconnect portion of the secondinterconnect dielectric layer 606 b; 2) simultaneously forming thesecond wire 608 b and second vias 610 b underlying the second wire 608 binset into the lower interconnect portion; 3) forming an upperinterconnect portion of the second interconnect dielectric layer 606 bon the second wire 608 b and the lower interconnect portion; and 4)forming a second vias 610 b overlying the second wire 608 b and insetinto the upper interconnect portion. Other processes for extending theinterconnect structure 604 are, however, amenable.

As illustrated by the cross-sectional view 1600 of FIG. 16 , adata-storage element 106 is formed overlying the interconnect structure604, on one of the second vias 610 b. The data-storage element 106 may,for example, be an MTJ, a MIM stack, or some other suitable structurefor data storage. In some embodiments in which the data-storage element106 is an MTJ, the data-storage element 106 comprises a referenceelement 202, a free element 204, and a barrier element 206. The barrierelement 206 is non-magnetic and is sandwiched between the reference andfree elements 202, 204. The reference and free elements 202, 204 areferromagnetic, and the free element 204 overlies the reference element202 and the barrier element 206. Alternatively, locations of thereference and free elements 202, 204 are switched.

In some embodiments, a process for forming the data-storage element 106comprises: 1) depositing a reference layer on the interconnect structure604; 2) depositing a barrier layer on the reference layer; 3) depositinga free layer on the barrier layer; and 4) patterning the reference,barrier, and free layers into the data-storage element 106. Otherprocesses are, however, amenable. For example, the free layer may bedeposited at 1) and the reference layer may be deposited at 3). Thedepositing may, for example, be performed by CVD, PVD, electrolessplating, electroplating, some other suitable deposition process(es), orany combination of the foregoing. The patterning may, for example, beperformed by a photolithography/etching process and/or some othersuitable patterning process(es).

As illustrated by the cross-sectional view 1700 of FIG. 17 , theinterconnect structure 604 is completed around the data-storage element106. The completed interconnect structure 604 comprises a thirdinterconnect dielectric layer 606 c, a third wire 608 c defining asource line SL, and a third via 610 c. The third interconnect dielectriclayer 606 c accommodates the third wire 608 c and the third via 610 c.Further, the third interconnect dielectric layer 606 c may, for example,be as the first interconnect dielectric layer 606 a is described. Insome embodiments, a process for completing the interconnect structure604 comprises: 1) depositing the third interconnect dielectric layer 606c; and 2) simultaneously forming the third wire 608 c and the third via610 c inset into the third interconnect dielectric layer 606 c. Otherprocesses for extending the interconnect structure 604 are, however,amenable.

The method illustrated by FIGS. 12-17 may, for example, be employed toform the memory cell in any one of FIG. 1, 2A, 2B, 3A, 3B, 4A, or 4B,the integrated chip in any one of FIG. 6A, 6B, 11A, or 11B, or thememory array in any one of FIG. 7, 8A-8C, 9A-9D, 10A, or 10B. Further,while the cross-sectional views 1200-1700 shown in FIGS. 12-17 aredescribed with reference to a method, it will be appreciated that thestructures shown in FIGS. 12-17 are not limited to the method and maystand alone without the method.

With reference to FIG. 18 , a block diagram 1800 of some embodiments ofthe method of FIGS. 12-17 is provided.

At 1802, an interconnect structure is partially formed on a substrate,where the partially formed interconnect structure comprises a bit linewire and pair of vias on the bit line wire. See, for example, FIG. 12 .

At 1804, a first unipolar selector and a second unipolar selector areformed neighboring respectively on the vias, where an anode of the firstunipolar selector faces the bit line wire and a cathode of the secondunipolar selector faces the bit line wire. See, for example, FIGS. 13and 14 .

At 1806, the interconnect structure is extended around the first andsecond unipolar selectors, where the extended interconnect structurecomprises an inter-selector wire electrically coupling a cathode of thefirst unipolar selector to an anode of the second unipolar selector.See, for example, FIG. 15 .

At 1808, a data-storage element is formed on the inter-selector wire.See, for example, FIG. 16 .

At 1810, the interconnect structure is formed around the data-storageelement, where the completed interconnect structure comprises a sourceline wire overlying and electrically coupled to the data-storageelement. See, for example, FIG. 17 .

While the block diagram 1800 of FIG. 18 is illustrated and describedherein as a series of acts or events, it will be appreciated that theillustrated ordering of such acts or events is not to be interpreted ina limiting sense. For example, some acts may occur in different ordersand/or concurrently with other acts or events apart from thoseillustrated and/or described herein. Further, not all illustrated actsmay be required to implement one or more aspects or embodiments of thedescription herein, and one or more of the acts depicted herein may becarried out in one or more separate acts and/or phases.

In some embodiments, the present application provides a memory cellincluding: a data-storage element having a variable resistance; and abipolar selector electrically coupled in series with the data-storageelement, wherein the bipolar selector includes a first unipolar selectorand a second unipolar selector, and wherein the first and secondunipolar selectors are electrically coupled in parallel with oppositeorientations. In some embodiments, a cathode of the first unipolarselector is electrically coupled to an anode of the second unipolarselector. In some embodiments, the first and second unipolar selectorare diodes. In some embodiments, the bipolar selector has a firstthreshold voltage at a first polarity and a second threshold voltage ata second polarity, and wherein the first and second unipolar selectorsindividually define the first and second threshold voltages. In someembodiments, the first and second threshold voltages are different. Insome embodiments, the data-storage element includes a MTJ, and whereinthe MTJ includes a reference ferromagnetic element and a freeferromagnetic element. In some embodiments, the free ferromagneticelement is electrically separated from the bipolar selector by thereference ferromagnetic element, wherein the reference ferromagneticelement is electrically separated from an anode of the first unipolarselector by a cathode of the first unipolar selector, and wherein athreshold voltage of the first unipolar selector is less than athreshold of the second unipolar selector. In some embodiments, thereference ferromagnetic element is electrically separated from thebipolar selector by the free ferromagnetic element, wherein the freeferromagnetic element is electrically separated from an anode of thefirst unipolar selector by a cathode of the first unipolar selector, andwherein a threshold voltage of the first unipolar selector is greaterthan a threshold of the second unipolar selector. In some embodiments, awidth of the first unipolar selector is different than a width of thesecond unipolar selector.

In some embodiments, the present application provides an integrated chipincluding: an array including multiple memory cells in a plurality ofrows and a plurality of columns, wherein the memory cells includeindividual bipolar selectors and individual data-storage elements, andwherein the bipolar selectors each include a first selector and a secondselector electrically coupled in parallel with opposite orientations;multiple first conductive lines extending along corresponding rows ofthe array and electrically couple with memory cells of the array in thecorresponding rows; and multiple second conductive lines extending alongcorresponding columns of the array and electrically couple with memorycells of the array in the corresponding columns. In some embodiments,the memory cells are MRAM cells. In some embodiments, the first andsecond selectors are unipolar selectors, wherein an anode of the firstselector is directly electrically coupled to a cathode of the secondselector, and wherein a cathode of the first selector is directlyelectrically coupled to an anode of the second selector. In someembodiments, the integrated chip further includes: a second arrayincluding multiple second memory cells in a plurality of rows and aplurality of columns, wherein the second memory cells include individualsecond bipolar selectors and individual second data-storage elements,and wherein the second conductive lines extend along correspondingcolumns of the second array and electrically couple with second memorycells of the second array in the corresponding columns; and multiplethird conductive lines extending along corresponding rows of the secondarray and electrically coupling with second memory cells of the secondarray in the corresponding rows, wherein the second conductive lines arevertically between the first conductive lines and the second conductivelines. In some embodiments, the first and second selectors are diodeshaving different threshold voltages in forward-biased states.

In some embodiments, the present application provides a methodincluding: providing a memory array including multiple memory cells inmultiple rows and multiple columns, wherein the memory cells include afirst memory cell, wherein the first memory cell includes a firstunipolar selector and a second unipolar selector, and wherein the firstand second unipolar selectors are electrically coupled in parallel withopposite orientations; applying a first voltage across the first memorycell at a first polarity, wherein the first and second unipolarselectors are respectively ON and OFF while applying the first voltageacross the first memory cell; and applying a second voltage across thefirst memory cell at a second polarity different than the firstpolarity, wherein the first and second unipolar selectors arerespectively OFF and ON while applying the second voltage across thefirst memory cell. In some embodiments, the applying of the firstvoltage sets the first memory cell to a first resistive state, andwherein the applying of the second voltage sets the first memory cell toa second resistive state that is different than the first resistivestate. In some embodiments, the memory cells further includes a secondmemory cell in the same row or column as the first memory cell, whereinthe second memory cell includes a third selector and a fourth selector,wherein the third and fourth selectors are electrically coupled inparallel with opposite orientations, and wherein the method furtherincludes: applying a third voltage across the second memory cell at thefirst polarity while applying the first voltage across the memory cellat the first polarity, wherein the third and fourth selectors are OFFwhile applying the third voltage. In some embodiments, the methodfurther includes: applying a fourth voltage across the second memorycell at the second polarity while applying the second voltage across thememory cell at the second polarity, wherein the third and fourthselectors are OFF while applying the fourth voltage. In someembodiments, the first unipolar selector has a threshold voltage betweenthe first and third voltages. In some embodiments, the first and secondunipolar selectors are diodes, and wherein the first and second unipolarselectors are each forward biased while ON, and wherein the first andsecond unipolar selectors are each reverse biased while OFF.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A memory cell comprising: a data-storage elementhaving a variable resistance and comprising a magnetic tunnel junction(MTJ), wherein the MTJ comprises a reference ferromagnetic element and afree ferromagnetic element; and a bipolar selector electrically coupledin series with the data-storage element and comprising a first unipolarselector and a second unipolar selector, wherein the first and secondunipolar selectors are electrically coupled in parallel with oppositeorientations; wherein the first unipolar selector has a differentthreshold voltage than a threshold voltage of the second unipolarselector and further has a different ON resistance than an ON resistanceof the second unipolar selector.
 2. The memory cell according to claim1, wherein the first unipolar selector has a lesser length than thesecond unipolar selector and further has a greater width than the secondunipolar selector.
 3. The memory cell according to claim 2, wherein thefirst and second unipolar selectors are diodes, wherein a cathode of thefirst unipolar selector is closer to the data-storage element than acathode of the second unipolar selector, and wherein the referenceferromagnetic element is closer to the first unipolar selector than thefree ferromagnetic element.
 4. The memory cell according to claim 1,wherein the first unipolar selector has a greater threshold voltage thanthe threshold voltage of the second unipolar selector and further has agreater ON resistance than the ON resistance of the second unipolarselector.
 5. The memory cell according to claim 4, wherein the first andsecond unipolar selectors are diodes, wherein a cathode of the firstunipolar selector is closer to the data-storage element than a cathodeof the second unipolar selector, and wherein the free ferromagneticelement is closer to the first unipolar selector than the referenceferromagnetic element.
 6. The memory cell according to claim 1, whereinthe first unipolar selector has a voltage increasing with current untila first current value is reached, wherein the voltage decreases ascurrent increases above the first current value until a second currentvalue is reached, and wherein the voltage increases with current abovethe second current value.
 7. A memory cell comprising: a data-storageelement having a variable resistance and comprising a magnetic tunneljunction (MTJ), wherein the MTJ comprises a reference ferromagneticelement and a free ferromagnetic element; and a bipolar selectorelectrically coupled in series with the data-storage element andcomprising a first unipolar selector and a second unipolar selector,wherein the first and second unipolar selectors are electrically coupledin parallel with opposite orientations and respectively comprise a firstinsulator and a second insulator; wherein the free ferromagnetic elementis separated from the bipolar selector by the reference ferromagneticelement, wherein the reference ferromagnetic element is separated froman anode of the first unipolar selector by a cathode of the firstunipolar selector, and wherein a thickness of the first insulator isless than a thickness of the second insulator.
 8. The memory cellaccording to claim 7, wherein the first and second unipolar selectorsare diodes.
 9. The memory cell according to claim 7, wherein the firstand second unipolar selectors have different ON resistances in forwardbiased states.
 10. The memory cell according to claim 7, wherein thesecond unipolar selector has a greater ON resistance than the firstunipolar selector in forward biased states.
 11. The memory cellaccording to claim 7, wherein the anode of the first unipolar selectorand the cathode of the first unipolar selector respectively underlie andoverlie the first insulator, and wherein an anode of the second unipolarselector and a cathode of the second unipolar selector respectivelyoverlie and underlie the second insulator.
 12. The memory cell accordingto claim 7, wherein the first insulator comprises doped hafnium oxide,such that the first unipolar selector has a snapback current-voltage(I-V) curve.
 13. The memory cell according to claim 7, wherein the firstand second insulators are semiconductor materials.
 14. A memory cellcomprising: a data-storage element having a variable resistance andcomprising a magnetic tunnel junction (MTJ), wherein the MTJ comprises areference ferromagnetic element and a free ferromagnetic elementvertically stacked; and a bipolar selector electrically coupled inseries with the data-storage element, wherein the bipolar selectorcomprises a first unipolar selector and a second unipolar selectorelectrically coupled in parallel with opposite orientations and havingdifferent ON resistances, wherein the first unipolar selector comprisesa first anode and a first cathode, and wherein the second unipolarselector comprises a second anode and a second cathode; wherein thefirst anode has a first orientation relative to the first cathode,wherein the second anode has a second orientation relative to the secondcathode, wherein an orientation of the reference ferromagnetic elementrelative to the free ferromagnetic element is a same as one of the firstorientation and the second orientation, and wherein a unipolar selectorcorresponding to the one of the first orientation and the secondorientation has a largest width amongst the first and second unipolarselectors.
 15. The memory cell according to claim 14, wherein thereference ferromagnetic element is oriented over the free ferromagneticelement, wherein the second anode is oriented over the second cathode,and wherein the second unipolar selector has the largest width.
 16. Thememory cell according to claim 14, wherein the first and second unipolarselectors have different threshold voltages, and wherein the unipolarselector corresponding to the one of the first orientation and thesecond orientation has a smallest threshold voltage amongst the firstand second unipolar selectors.
 17. The memory cell according to claim14, wherein the first unipolar selector comprises a first insulatorvertically stacked between the first anode and the first cathode,wherein the second unipolar selector comprises a second insulatorvertically stacked between the second anode and the second cathode,wherein the first and second insulators have different thicknesses, andwherein the unipolar selector corresponding to the one of the firstorientation and the second orientation has a smallest insulatorthickness amongst the first and second unipolar selectors.
 18. Thememory cell according to claim 14, wherein the free ferromagneticelement is oriented over the reference ferromagnetic element, whereinthe second anode is oriented over the second cathode, and wherein thefirst unipolar selector has the largest width.
 19. The memory cellaccording to claim 14, wherein the unipolar selector corresponding tothe one of the first orientation and the second orientation has asmallest height amongst the first and second unipolar selectors.
 20. Thememory cell according to claim 14, wherein the first anode, the firstcathode, the second anode, and the second cathode comprise metal.